Ducted fan gas turbine engines for aircraft conventionally comprise a large diameter propulsive fan which is mounted at the upstream end of the engine and is surrounded by an annular casing. During the operation of such engines, there is an extremely remote chance that a foreign body, such as a bird, could impact the fan and cause severe fan damage. That damage could result in part or all of a fan aerofoil blade becoming detached from the fan and impacting the fan casing. Such a detached fan blade could cause considerable damage to the aircraft powered by the engine if it were not contained by the fan casing. Similar damage could also result if the fan blade suffered failure through fatigue.
One way of ensuring that the fan casing contains such a detached fan blade is to manufacture the casing from a strong thick material such as steel. This unfortunately adds a large amount of undesirable weight to the engine. Alternatively the fan casing can be made from a thinner, lighter metal which is surrounded by a strong woven material as is described in GB 2159886B. Aromatic polyamide fibre is described as being suitable in the manufacture of the strong woven material.
The use of a lighter casing which is surrounded by a strong woven material is clearly attractive in view of the weight savings which it brings about. There is, however, still a danger that sharp pieces of detached fan blade could sever the woven fibre. Moreover, a thick layer of woven fibre is needed to provide effective fan blade containment.